1. Field of the Invention
This invention broadly relates to a flight crew oxygen mask having an extensible harness which is inflatable to enable the mask to be quickly donned, and then deflatable to permit the inherent resiliency of the harness to tightly urge the mask against the user's face over the nose and mouth area. More particularly, the invention concerns a valving arrangement for permitting limited reinflation of the harness when worn during certain flight conditions to increase the comfort of the wearer and relieve a portion of the tension of the harness holding the mask against the face.
2. Description of the Prior Art
An inflatable head harness for respirator devices is described and illustrated in U.S. Pat. No. 3,599,636 and comprises a mask that is connected to an elongated, extensible harness or strap having internal conduits connected by a valve to a source of pressurized air. When the valve is opened, air admitted to the conduits of the strap cause the strap to stretch and assume a somewhat rigid configuration. In this manner, the user can grasp the mask with one hand and direct the inflated strap behind his or her head, a particularly useful feature in an emergency situation for a flight crew when only one free hand is available.
Once the harness of the respirator shown in U.S. Pat. No. 3,599,636 is placed over the head, the strap is deflated and contracts in length. Thereafter, the inherent resiliency of the deflated strap urges the mask in tight engagement with the nose and mouth areas of the wearer's face in an attempt to avoid peripheral leakage of the breathable gas.
As a rule, flight crew masks must be pressurized when the aircraft is flying at cabin altitudes above approximately 40,000 feet in order to force air into the user's lungs. At these altitudes, therefore, the straps must exert a relatively large biasing force pressing the mask against the face to overcome the pressure of the oxygen urging the mask away from the skin and prevent oxygen leakage around the peripheral seal of the mask. However, at cabin altitudes of less than 40,000 feet, pressurized breathing conditions within the chamber of the mask are unnecessary and the regulator operates upon demand breathing such that an oxygen enriched air mixture is admitted to the mask only as the user inhales.
In general, the substantial majority of flight time is incurred at cabin altitudes at less than 40,000 feet. There are many situations, however, where the respirator mask must be worn at all times such as in cases where only one crew member is present. Therefore, the harness straps represent a substantial source of discomfort at lower altitudes when the respirator must be worn on the head at all times since the straps normally present a large degree of force even though pressurized breathing conditions are unnecessary.
The design and construction of flight crew respirators is subject to safety considerations as well as governmental regulations. In this regard, the respirator should be capable of being donned within few seconds in emergency situations with only one hand so that the remaining hand is free to operate the aircraft controls. As such, devices for relieving or increasing strap tension which require the use of two hands are completely unacceptable.